In the operation of hoists, it is highly critical that a high level of torque be immediately available at the initiation of a raising or lowering operation. Without high torque at the initiation of an operation, the load may not lift off the floor or, if suspended, control over it will be lost and it will drop to the floor. The loss of control over the load and its dropping, of course, may result in facility damage and injury to personnel.
Various protective measures for hoists utilizing adjustable frequency drives have been developed to protect against loss of load control or inability to lift a load due to insufficient torque. These include the sensing of hoist motor current and the determination that the current is at a level necessary to produce sufficient load controlling torque before the hoist brake is released as disclosed in U.S. Pat. No. 5,077,508, assigned to the assignee of the present invention. Another protective measure relates to decelerating a motor from frequencies exceeding 60 hertz as rapidly as possible and bringing the hoist to a stop. When decelerating quickly, the motor speed cannot be decreased so rapidly that the torque required from the motor exceeds the breakdown torque capability of the motor during deceleration. A solution for controlling the motor to decelerate as quickly as possible while at the same time producing sufficient torque is disclosed in U.S. patent application Ser. No. 07/726,494, filed Jul. 8, 1991, and assigned to the assignee of the present invention.
Another approach to preventing loss of load control, directed to increasing the torque that the motor is producing at the initiation of a hoist operation, is to increase the voltage supplied to the motor in excess of the voltage level that would normally be supplied by an adjustable frequency power supply at a low frequency start-up. The invention disclosed herein is a improvement in this type of hoist drive operation.